1. Field of the Invention
The present invention relates to a vehicle height control device for a vehicle such as an automobile or the like, and more particularly to a hydraulic circuit system therefor.
2. Description of the Prior Art
As is described, for example, in Japanese Patent Laying Open Publication No. 62-96112 (1987), typical vehicle height control devices for vehicles generally have such a construction as shown in FIG. 9 for each vehicle wheel. Stated more specifically, conventional vehicle height control devices include, in general, actuators 402 which are provided corresponding to the vehicle wheels of a vehicle and are adapted to increase and decrease the vehicle heights at locations corresponding to the vehicle wheels as respective results of the supply and the discharge of working fluid to and from their working fluid chambers 400; high pressure flow lines 406 which cooperate with connecting flow lines 404 to supply high pressure working fluid to the chambers 400; low pressure flow lines 408 which cooperate with the flow lines 404 to discharge the working fluid from the chambers 400; electromagnetic on-off valves 410 and electromagnetic flow rate control valves 412 provided in the high pressure flow lines 406; and electromagnetic on-off valves 414 and electromagnetic flow rate control valves 416 provided in the low pressure flow lines 408.
In the conventional vehicle height control devices having such a construction as mentioned above, when the vehicle height is to be adjusted to increase' the on-off valves 410 are opened and the valve opening of the flow rate control valves 412 is controlled so that a desired volume of working fluid is supplied to the working fluid chambers 400, while on the other hand when the vehicle height is to be adjusted to decrease' the on-off valves 414 are opened and the valve opening of the flow rate control valves 416 is controlled so that a desired volume of working fluid is discharged from the chambers 400. In the period, further, when working fluid high enough in pressure is not being fed to the high pressure flow lines as in a case where the engine of a vehicle which rotationally drives a hydraulic pump is not in operation, the on-off valves 410 and 414 are maintained in their closed position so as to prevent the vehicle heights from decreasing due to the leakage of working fluid from the chambers 400 through the flow lines.
The conventional vehicle height control devices are indeed effective for purposes intended but suffer from much consumption of electric power because two electromagnetic on-off valves 410 and 414 are provided for each vehicle wheel and these on-off valves must be controlled to open or close by controlling the electric current supplied thereto each time when the increasing or decreasing adjustment of the vehicle height is effected.
In addition, if any on-off valve 410 is opened so as to increase the vehicle height during the period when the pressure of the working fluid within and being fed to the high pressure flow line 406 is not high enough as in a case where the engine rotationally driving a hydraulic pump is not in operation, a portion of the working fluid in the associated working fluid chamber 400 will flow out of the chamber and will flow through the high pressure flow line in the opposite direction as expected, resulting in undesired decrease in vehicle height.
As the ways to solve the problem mentioned just above, may be suggested such an approach to incorporate a check valve in each high pressure flow line 406 which permits the flow of working fluid in the direction from a hydraulic pump not shown towards the associated working fluid chamber 400, or an approach to provide pressure sensors which detect the pressures in the high pressure flow lines 406 upstream of the on-off valves 410 and the pressures in the working fluid chambers 400, and to open the on-off valves 410 only during the period when the pressures in the high pressure flow lines are substantially higher than those in the associated chambers. The former approach, however, will cause to increase the load imposed on the pump and hence the engine, while the latter will render the system more expensive and its control more complex.
In the Japanese Utility Model Laying Open Publication No. 62-202404 (1987), an arrangement is described which has an on-off valve provided in the flow line connected between an actuator and an associated servo valve. In the arrangement, however, each on-off valve might have to be controlled in timed relation with the control of the associated servo valve and the electric power consumption would be increased if the former is an electromagnetic on-off valve, and it would be impossible to control at will the opening and closing of the on-off valve if the latter is a pilot-operated on-off valve which takes in as pilot pressure the pressure of the working fluid within the high pressure flow line and closes itself in response to the pilot pressure being not more than a predeterminate value.